The present invention relates to a method and equipment for negotiating a communication mode between system elements of a telecommunications system.
The first goal for circuit-switched and packet-switched telecommunications systems is to provide transmission means between end-users. Various kinds of interworking functions, such as rate adaptation, are needed between networks and terminals. Regardless of whether an underlying connection is circuit-switched or packet-switched, the system elements (involved in data transmission in networks or terminals) need to use common communication means before user data can be transferred. The usage of multi-mode system elements, that is, terminals and network elements supporting more than one communication mode, requires negotiation means for finding out the capabilities of the communicating elements. If the system elements are able to exchange information about the supported capabilities, the used communication mode (protocol), such as H.324 (Terminal for low-bit rate multimedia communication), may be selected. The communication mode selection may be done by in-band signalling, that is, utilizing the connection reserved for user data transmission, or by out-band signalling utilizing a separate signalling connection.
There are some interworking recommendations for multi-mode terminals, for instance the V.140 recommendation. V.140 is an ITU-T recommendation describing a standardized method for automatic communication mode negotiation, detection of bit alignment, and confirmation of sub-channel connectivity for multiprotocol audiovisual terminals on digital networks such as the ISDN (Integrated Services Digital Network).
V.140 procedures consist of three phases and begin following the establishment of an end-to-end digital connection before any multimedia or other communication protocols are initiated. V.140 procedures can also be used to provide an optional initial voice telephony mode before proceeding to multimedia telephony, and to switch from one multimedia telephony mode to another, or back into voice telephony mode. The V.140 procedures concern the flow of signals along the fixed digital paths at integer multiples of 64 kbit/s (or 56 kbit/s in certain networks). The means by which digital channels are established are outside the scope of V.140.
In general, an originating network, that is, the network the call is made from, does not know the capabilities of the terminating network or which communication mode to use. Thus, errors and disconnections may occur, if a system element of the terminating network does not support the desired communication mode, for example a multimedia call. Out-band signalling may be used, however, it is difficult to introduce new out-band signalling to existing systems. In-band signalling may also be faster, which may be needed in some cases, for example when a connection is changed from one base station to another (handover). It is also possible to use V.140, but there are communication modes that are not supported by V.140. V.140 is not very commonly used and V.140 procedures can not be used to negotiate the communication mode if the other end does not support V.140.